The key to quantifying chemical weathering intensity: clay stable isotope fractionation factors

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

Chemical weathering is the process by which rocks dissolve in contact with natural waters. This process of dissolution supplies nutrients to plants, solutes to waters (of the kind you might find on the label of a bottle of mineral water) and delivers nutrients to the oceans via rivers and groundwaters. These nutrients are used by algae and microscopic plankton to grow, including the growth of shells of marine fauna which are made from calcium carbonate. This coupled process of mineral dissolution on the continents and biogenic calcium carbonate formation in the oceans plays a key role in regulating the global carbon cycle, transferring carbon from the atmosphere to the carbonate rock reservoir, when shells become buried as sediments. Geologists think that this simple series of reactions has played a pivotal role in regulating the carbon cycle and hence climate over Earth's history.

Critical to this series of processes are clays. Rocks don't simply just dissolve in rainwater, they break down slowly, and their transformation into solutes happens in a stepwise manner via the formation of new minerals, frequently clays. Clays are a very common mineral and present in soils and rocks in the world all around us. We seek to use novel chemical methods to understand how clays are important to the processes of chemical weathering, exploiting recent technological advances that allow us to measure very precisely isotope ratios of the key elements in clay formation, magnesium, silicon and lithium. Different isotopes of an element, have very small differences their mass because of additional neutrons in their nuclei. Geochemists can measure small differences in the amount of these isotopes and from these differences infer key points about how the clays are being made. Importantly these isotopes are fractionated when clays crystallise in the natural world, but this is poorly understood. To improve our understanding, we seek to make artificial clays in the laboratory, where we can control all the conditions of formation carefully, and observe how the isotopes behave. With our detailed understanding of how the isotopes behave in the laboratory, we will then apply this knowledge to the natural world, where we will gain a much better understanding of chemical weathering, and how the carbon cycle, and hence climate has behaved over Earth history.

Planned Impact

There will be numerous beneficiaries from this research including scientific, social, economic and environmental beneficiaries:

Scientific Impact
The scientific impact is outlined in more detail in the section "academic beneficiaries" but in summary, the scientific impact will be broad across the spectrum of Earth system and environmental science, including:
1. Geochemists,
2. Soil scientists,
3. Paleaoclimatologists,
4. Global biogeochemical cycle modellers.

Research led undergraduate teaching
There is a strong reputation of research led teaching at St Andrews, and Tipper currently lectures environmental geoscience to third year undergraduates, with a focus on chemical weathering and the chemistry of natural waters. This grant will provide the opportunity to engage undergraduate students (the majority of which gain in employment in industry rather than academia) in the problems of natural water chemistry including the key results of the research project about clay formation. This grant will also provide the perfect opportunity to engage undergraduates with the use of isotope tracers, including training in the analytical protocols, adding significant value to their BSc. They will even be given the opportunity to interpret the project data in their lab practicals, and potentially be encouraged to work on similar topics for their dissertations or the new masters degree beginning at St Andrews.



Commercial Impact
The commercial impact of understanding and constraining clay formation is significant. The pivotal roles that clays play in the permeability of hydrocarbon reservoirs and aquifers, and the pressure on hydrocarbon and water resources means that there is a very strong interest in clays from industry. We already have links with industry at St Andrews to better understand clays in hydrocarbon reservoirs.




Impact to the public and future generations of scientists
Our growing area of impact is with outreach, involving academics, students, and early career researchers in direct engagement with school pupils and teachers, and the public. Our schools outreach programme (GeoBus) is innovative and has broad and wide ranging impact (over 20000 pupils in 160 different schools since January 2012, and much media coverage). NERC originally funded the project (£50k) and total funding to 2016 is now >£350k with industry sponsors. It is also a mechanism for science engagement with the public in science centres (e.g. Our Dynamic Earth and Sensation) and at festivals (e.g. British Science Festival, September 2012). GeoBus is already a successful "vehicle" for communicating RCUK science and the learning activities are built around research outcomes.



Soil (agriculture and natural ecosystem)
At a time of rapid environmental change our understanding of clays and the soil environment is particularly important. Understanding the fundamentals of how clays regulate the release of nutrients both in the world of agriculture and natural world is essential to understand the sustainability of the biosphere, and how the biosphere and agriculture will respond to a changing environment.
 
Description We have developed a method for reproducibly synthesising clay minerals (smectite and stevensite) under low temperature (<50 degC) conditions. Importantly, we are able to synthesise enough material to allow for full solid state characterisation and measurement of the isotopic composition. Our results confirm previous low temperature laboratory experiments that light Mg and Li is preferentially incorporated into clay minerals and that this process is pH and temperature dependent. Additionally, we have discovered that the clay exchangeable pool is an important reservoir for Li and Mg and therefore an important factor determining the bulk clay isotopic composition.
Exploitation Route Clay formation is thought to be one of the main controls of riverine Mg and Li compositions. Our results show that in addition to the conditions under which the clay forms (pH, T etc.), the size of the exchangeable pool also has a key control on the bulk isotopic composition of the synthesised clay. This result may help resolve the present discrepancy regarding the direction of fractionation during low temperature clay formation between lab and field Mg isotope data and therefore lead to an improved understanding of Mg biogeochemistry.
Sectors Education,Environment
 
Description NERC Urgency grant: Himalayan chemical weathering and carbon transfer triggered by the April 2015 Nepal Earthquake
Amount £51,818 (GBP)
Funding ID NE/N007441/1 
Organisation Natural Environment Research Council (NERC) 
Sector Public
Country United Kingdom of Great Britain & Northern Ireland (UK)
Start 07/2015 
End 07/2015